A conventional photovoltaic cell incorporates a semiconductor structure with a junction between semiconducting materials with different majority-carrier conductivity types, such as a p-n junction formed between an n-type semiconductor and a p-type semiconductor. More specifically, crystalline Si photovoltaic cells are typically made by adding controlled impurities (called dopants) to purified silicon, which is an intrinsic semiconductor. Dopants from IUPAC group 13 (e.g., B) are termed “acceptor dopants” and produce p-type material, in which the majority charge carriers are positive “holes,” or electron vacancies. Dopants from IUPAC group 15 (e.g., P) are termed “donor dopants” and produce n-type material, in which the majority charge carriers are negative electrons. Dopants may be added to bulk materials by direct inclusion in the melt during silicon crystal growth. Doping of a surface is ordinarily accomplished by providing the dopant at the surface in either liquid or gaseous form, and then thermally treating the base semiconductor to cause the dopant atoms to diffuse inward. Ion implantation, possibly with further heat treatment, is also used for surface doping. In some implementations, the n-type material and/or the p-type material can each respectively be comprised of regions having varying levels of relative dopant concentration. Regions having enhanced amounts of dopant are conventionally denoted as n−, n+, n++, and p−, p+, p++, indicating regions in order of increasing doping levels.
When the cell is illuminated by electromagnetic radiation of an appropriate wavelength, such as sunlight, a potential (voltage) difference develops across the p-n junction as the electron-hole pair charge carriers migrate into the electric field region of the junction and become separated. The spatially separated charge carriers are collected by electrodes in contact with the semiconductor at one or both surfaces. The cell is thus adapted to supply electric current to an electrical load connected to the electrodes, thereby providing electrical energy converted from the incoming solar energy that can do useful work. Since sunlight is almost always the light source, photovoltaic cells are commonly known as “solar cells.” Ideally there is a low resistance connection between each electrode and the associated device and the electrode itself has high electrical conductivity, so that the efficiency of the source in converting incident light energy to usable electrical energy is maximized, with minimal ohmic losses within the device.
Industrial photovoltaic cells are commonly provided in the form of a planar structure, such as one based on a doped crystalline silicon wafer, that has been metallized, i.e., provided with electrodes in the form of electrically conductive metal contacts through which generated current can flow. Most commonly, these electrodes are provided on opposite sides of a generally planar cell structure. Conventionally, they are produced by applying suitable conductive metal pastes to the respective surfaces of the semiconductor body and thereafter firing the pastes to form a thin metal layer.
In the common planar p-base configuration, a negative electrode is located on the side of the cell that is to be exposed to a light source (the “front,” “light-receiving,” or “sun” side, which in the case of an ordinary solar cell is the side exposed to sunlight); a positive electrode is located on the other side of the cell (the “back” or “non-illuminated” side). For example, many commercial cells are constructed using a 200 μm thick p-type Si wafer with a 0.4 μm thick layer of n-type Si layer at the front surface. The p-type wafer is the base. The n-type layer is the emitter and is made by either diffusion or ion implantation of phosphorus (P) dopant into the Si wafer. Cells having a planar n-base configuration, in which the p- and n-type regions are interchanged from the p-base configuration, are also known. Solar-powered photovoltaic systems are considered to be environmentally beneficial in that they reduce the need for burning fossil fuels in conventional electric power plants.
Photovoltaic cells are commonly fabricated with an insulating layer on their front-side surface to afford an anti-reflective property that maximizes the utilization of incident light. However, in this configuration, a portion of the insulating layer normally must be removed to allow the overlaid front-side electrode to make contact with the underlying semiconductor. Conductive metal pastes appointed for fabricating front side electrodes typically include a glass frit and a conductive species (e.g., silver particles) carried in an organic medium that functions as a vehicle for printing. The electrode may be formed by depositing the paste composition in a suitable pattern (for instance, by screen printing) and thereafter firing the paste composition and substrate to dissolve or otherwise penetrate the insulating, anti-reflective layer and sinter the metal powder, such that an electrical connection with the semiconductor structure is formed.
The specific formulation of the paste composition has a strong but highly unpredictable effect on both the electrical and mechanical properties of electrodes constructed therewith. To obtain good electrical characteristics for the finished cell (e.g., high light conversion efficiency and filling fraction and low contact and source resistances), the composition must penetrate or etch fully through the anti-reflective layer during firing so that a good electrical contact is established, but without damaging the underlying semiconductor. However, it is also desired that a strongly adhering bond between the electrode and the substrate is formed upon firing. With many conventional paste compositions, it has not proven possible to reliably fire the printed wafers so that good adhesion and electrical properties are obtained concomitantly.
Although various methods and compositions useful in forming devices such as photovoltaic cells are known, there nevertheless remains a need for compositions that permit fabrication of patterned conductive structures that provide improved overall device electrical performance and that facilitate the rapid and efficient manufacture of such devices in both conventional and novel architectures.